This invention relates to an improved scrap reclamation system and in particular to a method and apparatus for quickly and efficiently melting metal scrap in a recirculating melting system.
Increased effort is being placed on finding cheaper methods to recycle metal scrap, particularly in the light metals field. This effort is being expended because of the national and international concern over depletion of raw material sources for these metals.
Recycling systems for metal scrap are normally required to be at least competitive with established manufacturing systems for producing such metal. To be competitive, such recycling systems must be efficient which can often require that they be designed to reclaim certain or specific kinds of metal scrap. With respect to the kinds of metal scrap encountered in the light metals field, particularly aluminum, it should be observed that it can range from scrap generated very early in the manufacturing process to that resulting from used products, e.g. metal containers. Scrap generated early in the manufacturing of aluminum, for example, is scalping chips. These chips result from preparing the surface of an aluminum ingot for rolling or cladding or some such purpose when often large irregularities are removed. Chips removed at this time can constitute as much as 6% of the total metal in the ingot and, therefore, obviously can amount to a considerable amount of metal which can be recycled. Scrap typical of used metal containers which can be beneficial to recycle is metal beverage cans and the like. Recycling such cans serves to conserve resources and also to provide a cleaner environment.
These two types of scrap are often considered to present different problems from the standpoint of reclamation. For example, container or can scrap normally comprises bits and pieces of metal which can range in size from relatively small particles to crushed cans. Often, the cans can have holes pierced in them for purposes of delacquering using solvents. The pierced holes, while aiding the flow of solvent through the mass of cans, result in jagged edges which, together with the generally non-uniform shapes and sizes of crushed cans, make this scrap relatively non-flowable. Furthermore, such scrap tends to be rather buoyant with respect to the melting media, requiring special equipment to forcibly submerge it. A particular type of equipment for handling this type of scrap is disclosed in U.S. Pat. No. 3,873,305 where a rotating wedge forcibly submerges this type of scrap into the melting media. In the system described in this patent, additional equipment is required to recirculate the molten melting media.
U.S. Pat. No. 4,128,415 illustrates another system for reclaiming scrap wherein the melting media is drawn up into a bay where a metal scrap is melted. Further, U.S. Pat. No. 3,997,336 discloses a system where melting media and metal scrap are introduced into a bay and the scrap is forcibly submerged using an axial flow impeller.
With respect to the scalping chips, or scrap which may be characterized as flowable, for example, such as filings and cuttings or even large pieces, these can present difficulty in submerging as does the above-mentioned container scrap. Nevertheless, it is imperative that such flowable scrap be melted and reclaimed in the most efficient manner in order to provide an incentive to operate a remelting system as a source of metal, as compared to the established manufacturing system for producing such metal.
With highly oxidizable metals, such as aluminum, out of which the aforementioned containers and beverage cans are often made, it is important to provide a remelting system which provides maximum heat transfer for efficient melting. Also, it is important that the metal scrap be submerged with minimum turbulence at the melting media surface in order to minimize oxidation of the molten metal. Oxidation leads to skim formation, a problem inherent in most systems reclaiming highly oxidizable metals such as aluminum.
The skim referred to includes the metal oxide, e.g. Al.sub.2 O.sub.3, and molten metal which becomes entrained in such oxide. Skim is undesirable and, therefore, should be minimized not only because of the molten metal lost to it, but also because it can cause problems by acting as an insulator in the heating bay. That is, when combustion units discharging their heat on the surface of the molten media in the heating bay are used, skim floating on the surface thereof makes it difficult to heat the molten media. Also, such floating skim by rejecting heat can seriously shorten the useful life of the equipment by causing it to overheat.
The present invention provides a highly efficient method of recirculating molten melting media and for reclaiming metal scrap in a recirculating melting system. The method is suitable for reclaiming either flowable or non-flowable scrap of the type referred to above.